AU2020101106A6 - A controlled-source audio-frequency magnetotellurics method for prospecting deeply buried resources - Google Patents

Abstract

This invention relates to a controlled-source audio-frequency magnetotellurics method for prospecting deeply buried resources. The method comprises: emitting electromagnetic waves by using transmitter with a fixed current and preset power and obtaining data on electromagnetic wave through observation equipment profile or matrix; processing electromagnetic wave data by applying the full-space electromagnetic wave propagation model derived from the attenuation feature of the electromagnetic field in the diffusion medium; completing a fine detection and attaining detailed geological information of a target ore through data analysis based on the transient electromagnetic field theory. The invention can be applied in deep underground resources exploration, thereby identifying both geological electrical and structural information. transmitter moving drcton of transmiter r>4-6 H /A receiver Figure. 1 7000 6500 6000 5500 5000 ene 4500 4000 - 3500 3000 2500 2000 1500 1000 500 0 \% -500 Figure. 2 1

Description

transmitter

moving drcton oftransmiter

r>4-6 H /A

receiver

Figure. 1

7000 6500 6000 5500 5000 ene 4500 4000 - 3500 3000 2500 2000 1500 1000 500 0 \% -500

Figure. 2

A CONTROLLED-SOURCE AUDIO-FREQUENCY MAGNETOTELLURICS METHOD FOR PROSPECTING DEEPLY BURIED RESOURCES

Field ofthe Invention

The invention relates to the field of electromagnetic geophysical exploration. More particularly, this invention relates to a controlled-source audio- frequency magnetotellurics method for prospecting deeply buried resources.

Background of the Invention

According to a worldwide assessment of near-surface resources, a rich array of geological resources is under the earth's surface in various countries and regions, yet the exploration rate worldwide stays relatively low. The latest assessment on petroleum suggests that China's petmleum reserves are about 100 billion tons in total, of which 21.29 billion tons have already been explored. This implies that approximately 72.5% of petroleum is yet to prospect. On the other hand, natural gas reserves nearly 77.51 million tons in total with 93.58% under prospecting. A similar situation also exists in metal mines. There are fewer outcmps and shallow deposits, which require detailed exploration of larger-scale and extra larger-scale deep deposits. It is an urgent yet strategic task to detect deep-seated oil and gas and mineral deposits.

Electromagnetic methods have proven to be effective tools in the exploration of deeply buried resources and targets. Wherein, the contlled-source audio- frequency magnetotellurics method (CSAMT) is commonly used electromagnetic appmaches, it plays an important role in detecting deep-seated oil and gas and mineral deposits. Referring to FIG. 1, CSAMT has been widely applied in surveys of mineral prospecting and hydrology. The transmitter length used in CSAMT ranges fmm 1km to 3km (Surface and formation wave, Offset: ~10km, Depth: ~1km, Half-space model), the transmitting power is usually less than 30 kW, and the transmitting current is 20A. Worth noting, the electromagnetic signals with various frequencies are usually excited underground. In general, the electromagnetic waves emitted are square waves with preset frequency, and response signals sending out from the underground target body are captured at a certain distance away from the transmitting source. When applying CSAMT, one or more observation equipment is applied to process signals captured and attain detailed geological information of the target body. Due to its relatively short processing time, high efficiency, strong anti- interference ability, and ability to reach 1km detection depth, CSAMT has been widely used in the shallow exploration of mineral general survey, hydrology and engineering environment

Although the CSAMT method is often considered as an advanced active-source electromagnetic method for deeply buried earth resources, the effective observation area of CSAMT is often limited to the range of 5-10km away from the target due lo the near-field effect and the controlled-source souce. To cover the observation area entirely, transmitting source set on the long observation profile needs to be re arranged multiple times till it is in parallel with the survey line. Consequently, this not only rduces the work efficiency, but also may result in the field source effect, making the signal consistency poor and not conducive to a fine detection. Secondly, CSAMT is not an ideal method to be used in mountain areas as heavy transmitter devices make it difficult to transport and pole locations make it hard to locate. Thirdly, due to the limitation of transmitting power, the observation range is limited to <10km. Several deep tunnels need to be excavated, multi- layer aluminum platinum and salt water need to be embedded in order to ensure the transmitting dipoles have good contact with the ground, thus reducing the grounding resistance and maximizing the emission current. Finally, it takes significant amount of manpower and time to establish transmitting antenna with 1-3km in length especially in mountainous areas. In sum, the traditional CSAMT as a small-scale active-source electromagnetic method, is not competent for tasks like deep and precise earth exploration.

Previously, searchers from the United States and the Soviet Union have attempted to use wireless artificial source electromagnetic fields for detecting deep seated deposits. However, the results of their theoretical or applied research have not been published orreported as no deep-seated deposits were built for detecting.

Consequently, the problem of how to overcome the limitations in detection depth, resolution, and signal using CSAMT, and to achieve high-quality detection, needs to be answered and resolved.

Summary of the Invention

The invention disclosed aims to address the underlying technical problem by introducing an electromagnetic method for prospecting deeply buried resources, which can be applied to deep resource detection, thereby identifying and obtaining detailed geological and structural information of the target body.

The invention discloses a controlled-source audio- frequency magnetotellurics method for prospecting deeply buried resources. The method comprises the following steps:

Emitting electromagnetic waves by using transmitter with a fixed curont and

preset power, and obtaining data on electromagnetic wave through observation equipment profile or matrix;

Processing electromagnetic wave data by applying the full- space electromagnetic wave propagation model derived from the attenuation featum of the electromagnetic field in the diffusion medium;

Completing a fine detection and attaining detailed geological information of a target ore through data analysis based on the electromagnetic field theory.

In a preferred embodiment of the present invention, each electric and magnetic field components of the full-space electromagnetic wave propagation model can be expressed as:

E =- P0l0IdscosY H S2F hD 21 n In

E = pg Ids i s | 00 H A (z)F n+sn H F 1

ds0 H 1n n m 2m E 1Wp dsi s H z+zH F2

o L nI

Ids SIin(p 2 H =H S F, r hD R

HO= -Ids isinTp 0 IH F + YH F,n

H, = hD nko D[I | L sin0 HnFn n + m ZYHm_ An ( z)| I_

Ids icosp F,n hD 7k D n2 sin n A FZ

F = Oc 0 P (-cos0) F = 021 .2 P (-cos0) where: iq 2sin(vR)00 ' V2q 2sin(vnt)a02 V

H =A (z)S- 2AG (z ) G (z), H =S- 2 A G (z ) G (z) n n n n n s n m m m m S m

D=aO, P is the Legendre polynomial function, A,=Zg / ye/s7

A,= Z, / p, /, , H, , are the spherical harmonic coefficient;

In the formulas indicated above, A , G, A , S presents the excitation

factors of TM mode and TE mode (with subscript m), high gain function, high

normalized sensitivity and propagation factor respectively. Among which, TM mode has subscript n, TE mode has subscript m, the propagation factor is

S= C / V -i5.49a / f . In addition, C is the propagation speed of electromagnetic

wave in vacuum 3.0x10 8 m/s, V is the propagation phase speed of electromagnetic wave, f is the electromagnetic wave frequency, a is the attenuation rate of

electromagnetic wave in the earth ionosphere waveguide, o is the permeability, 6o

is the susceptibility, I is the emission current, ds is the length of the emission dipole,

# is the azimuth of the measuring point calculated from the x-axis, 0 is the angle

between the emission dipole ds and the vector direction of field point. In a preferred embodiment of the present invention, the transmitter with a fixed current and preset power used to emitted electromagnetic waves consists of two transmitting devices:

Each transmitting device can emit electromagnetic waves with frequency range of 0.1---300Hz, the length of transmitting antenna is approximately 100 kin, power is at 500-1000kW, and current is at 250A.

In a preferred embodiment of the present invention, one or more observation equipment profile or matrix to be applied, and each observation equipment prfile or matrix to include N number ofobservation devices:

The N number of observation equipment are set to be fixed with dipoles in the both east-west direction and south-north direction. The positive direction of the x axis is horizontal to the north, the positive y axis is horizontal to the east, the positive z axis is downwards vertical. Each observation equipment collects data fom three sounding points simultaneously. Whemein, the middle sounding point measures two electric field components and three magnetic field components, while the sounding points at both ends only measure two electric field components. The observation equipment introduced has 12 single channels, including three magnetic channels and nine electrical channels.

In a preferred embodiment of the posent invention, the performance parameters ofthe 12single channels of observation equipment are as follows:

There are 24 frequency sampling points, where the lowest sampling frequency is 24000Hz and the highest is 600 kHz; The bandwidth is DC-i0kHz, the dynamic range is >130dB, the input impedance is >1OMQ, and the standard OOMbit twisted-pair cable is used for network connection; The channels support USB 1.1, USB 2.0, wireless, Bluetooth technology, and the synchronization mode is GPS clock, UTC± ns; The internal crystal vibration is <±5x10-9,and thepower consumption when 12 channels working simultaneously should be less than 12W.

Compared with prior techniques, the invention disclosed has the following advantages:

Firstly, the invention discloses a new method for deep earth exploration. Using a large-scale controlled transmitting antenna with a length of 100 km and power level reaching megawatt, the distance between the transmitter and receiver is able to be expanded fom 10 km to more than 3000 km. Additionally, the observation range is also able to be expanded, resulting in electromagnetic detection covering the entire national territory and the adjacent sea areas.

Secondly, the form of controlled-source observation is adopted in this invention disclosed. As a result, the captured signal is 20-40dB stronger than the active-source method. Worth noting, the performance of controlled-source method is generally stronger in signal intensity, better in single consistency, higher in the signal-to-noise ratio, and more precise in detection profile comparing to traditional active-source profile. On the other hand, the information obtained on the targeted metal ore is consistent with the actual geological data. Comparing to the CSAMT method, the disclosed method is able to complete a fine detection and attain detailed geological information of the target body.

Last but not least, as a new deep earth exploration method, the invention will have great potential in exploration of metal om and resources such as petroleum and gas.

Other features and advantages of the invention disclosed will be described in a subsequent section. The purpose and other advantages of the invention can be obtained and understood fom the structure indica ted in the detail description, the claims and the referring figures.

Brief Description of the Drawings

The rferring figures are used to provide a further understanding of the technical solutions of the disclosed invention, and form a part of the specification. Together with the embodiments of the invention, the referring figures serve the purpose of explanation, yet they do not constitute limitations on the technical solutions of the invention.

FIG. 1 is a simplified pictorial representation of the Controlled-source Audio frequency Magneto-telluric Method;

FIG. 2 is a simplified pictorial representation of the ground electromagnetic detection embodying in the present invention;

FIG. 3 is a simplified flowchart of the ground electromagnetic detection embodying in the present invention;

FIG. 4 is a simplified pictorial representation of 3-D electromagnetic wave propagation model embodying in the present invention;

FIG. 5 is a simplified pictorial representation of a contour map of an ore body roof embodying in the present invention;

FIG. 6 is a simplified pictorial representation of WEM (Wireless Electromagnetic Method) line measurement embodying in the present invention;

FIG. 7 is a simplified pictorial representation of receiver's measurement pole embodying in the present invention, where Ex is pointing north and south, and Ey is pointing east and west;

FIG. 6 is a simplified pictorial representation of WEM (Wireless Electromagnetic Method) line measurement embodying in the present invention;

FIG. 7 is a simplified pictorial representation of receives measurement pole embodying in the present invention, whem Ex is pointing north and south, and Ey is pointing east and west;

FIG. 8 is an electric field vector propagation diagram embodying in the present invention; wherein, FIG. 8(a) is an electric field vector prpagation diagram calculated using the CSAMT method, and FIG. 8(b) is an electric field vector propagation diagram calculated using the controlled-source method with high power;

FIG. 9 is an electromagnetic field spectrum diagram of a full- frequency point series measured at 2300m depth of 2 measuring lines embodying in the present invention; wherein, FIG. 9(a) is a spectrum curve of Ex, FIG. 9(b) is a spectrum curve of Ey, FIG. 9(c) is the spectrum curve of Hx, FIG 9(d) is the spectrum curve of Hy, FIG. 9(e) is the spectrum curve of Hz;

FIG. 10 is a graph of the electromagnetic field observed at 2300m depth of 2 measuring lines embodying in the present invention; wherein, FIG. 10(a) is the electric field amplitude diagram, FIG. 10(b) is the magnetic field amplitude diagram, FIG.10(c)is the apparent resistivity curve, FIG. 10(d)is the impedance phase curve;

FIG. 11 is a pseudo cross-sectional view of the original apparent resistivity and impedance phase of the 3 measuring lines in the xy direction embodying in the posent invention; FIG. 11(a) is the crss-sectional view of the original apparent resistivity in the xy direction, FIG. 11(b) is the impedance phase pseudo-section in the xy direction;

FIG. 12 is a comprehensive diagram of WEM three-dimensional inversion result embodying in the present invention;

FIG. 13 is a comparison diagram of the results of molybdenum ore detection inversion embodying in the present invention; wherein, FIG. 13(a) is the inversion result using the CSAMT method, FIG. 13(b) is the inversion result using thelarge scale controlled-source method, FIG. 13(c) is a geological data map.

Detailed Description of the Embodiments

To ensure a clear illustration of the objective, technical scheme and advantages of the present invention, a detailed description of the invention embodiments is given below in combination with refermd figures. Worth noting, the embodiments in the present invention and the features in the embodiments can be arbitrarily mixed without conflict.

The steps illustrated in the rferred flowchart can be performed in an executable computer system. Despite the fact that the logical sequence is shown in the flowchart, described steps can be performed in a different order in some cases.

Referring to FIG. 2, a simplified pictorial repoesentation of the ground electromagnetic detection embodying in the present invention was shown. This includes the transmitter with a fixed current and preset power, as well as the rceiver. Each transmitting device can emit electromagnetic waves with frquency range of 0.1--300Hz, the length of transmitting antenna is approximately 100 km, power is at 500-1000kW, and current is at 250A.

Since the current in the high resistance area can penetrate to a large depth, two 500kW transmitting equipment can transmit electromagnetic waves with high intensity covering the entire national territory. This enables the detection depth reaching up to 10km while maintaining the observation resolution captured using traditional active-source electromagnetic method.

When transmitting electromagnetic waves, 30 observation equipment am used to form the observation pmfile or observability matrix so that relatively dense electromagnetic wave data can be obtained. (Theoretically speaking, any number of observation equipment can be used to observe simultaneously, and the number of observation equipment can be increasedflexibly)

In the embodiment of the invention, the disclosed controlled-source audio frequency magnetotellurics method has the following characteristics:

(1) The observation range is not limited to a small region; hence, it is suitable method for large-scale detection;

(2) The proposed method does not require to carry heavy transmitter devices; hence, it is easy to conduct field work in mountain areas;

(3) The proposed method does not require to move the transmitter several times within one survey area; hence, it helps to increase the work efficiency.

Referring to FIG. 3, the controlled-source audio- frequency magnetotellurics method embodying in the present invention is presented. The method comprises the following steps:

S101: Emitting electromagnetic waves by using transmitter with a fixed current and preset power, and obtaining data on electromagnetic wave through observation equipment profile ormatrix;

S102: Processing electromagnetic wave data by applying the full-space electromagnetic wave propagation model derived from the attenuation featum of the electromagnetic field in the diffusion medium;

S103: Completing a fine detection & attaining detailed geological information of a target ore through data analysis based onthe electromagnetic field theory.

In the embodiment of the invention, a theoretical full-space electromagnetic wave propagation model based on active-source electromagnetic detection method (shown in FIG. 4) is established. Meanwhile, both conduction current and displacement current are taken into account in the entire space of the ionosphere atmosphere-rock layer. It is generated by injecting hundreds of amperes of current into the underground using a 100 km long ground wire. Then, it propagates upward to the ionosphere, and reflected by the ionosphere back to the ground again. The electromagnetic field signal carried by the underground target can be detected using the layer matrix and the R function.

In the embodiment of the invention, each electric and magnetic field components of the full-space electromagnetic wavepropagation model can be expressed as:

E =-4o1-0IdscosY H S2F r hD 21 n In

EO = p dsip A + s0 H F

o L I

E,-- 9r d i1H/, H F2m| o L n M

Ids sip H 2V hD

, isinp HF + H F27 H19= -Ids

hD nk D Lsin0n m An (z)|

Ids icosLp HF + H F H = hD kD n 2" sinO n A (7)

F = 7 P (cosO) F 0 2R ,2 P (-cos) where: Iq 2sin(vR)a0 V 2q 2sin(V7)02 v

H =A (z)S-2AG (z) G (z), H =S-2 A G (z ) G (Z) n n n n n S n

D=aO, P, is the Legendre polynomial function, Jg=Zg/ p1/s7

, A = Z' /ipZ /c, , Hn , IIn are the spherical harmonic coefficient;

In the formulas indicated above, A , G , A , S presents the excitation

factors of TM mode and TE mode (with subscript m), high gain function, high normalized sensitivity and propagation factor respectively. Among which, TM mode has subscript n, TE mode has subscript m, the propagation factor is

S = C / V -i5.49a / f . In addition, C is the propagation speed of electromagnetic

wave in vacuum 3.0x10 8 m/s, V is the propagation phase speed of electromagnetic wave, f is the electromagnetic wave frequency, , is the attenuation rate of

electromagnetic wave in the earth ionosphere waveguide, /o is the permeability, o

is the susceptibility, I is the emission current, ds is the length of the emission dipole,

# is the azimuth of the measuring point calculated from the x-axis, 0 is the angle

between the emission dipole ds and the vector direction of field point.

The embodiment of the invention has addressed the strong boundary problem caused by the significant difference between the cross-scale division of the model and the electric property between the layers. It has deduced the exact expression of the response electric field of the sky wave Ex and clarified the mechanism of the full space, slow attenuation and long-distance waveguide propagation of the sky wave. In addition, it has also illustrated the propagation rule, in which the lower the frequency is, the farther the distance into the waveguide area is, and changed the traditional understanding of the attenuation character of the sky wave according to the third power of the distance.

In the embodiment of the invention, the transmitter with a fixed current and preset power used to emitted electromagnetic waves consists of two transmitting devices:

Each transmitting device can emit electromagnetic waves with frequency range of 0.1-30Hz, the length of transmitting antenna is approximately 100 km, power is at 500-1000kW, and current is at 250A.

In the embodiment of the invention, one or more observation equipment profile or matrix to be applied, and each observation equipment profile or matrix to include N number of observation devices:

The N number of observation equipment am set to be fixed with dipoles in the both east-west direction and south-north direction. The positive direction of the x axis is horizontal to the north, the positive y axis is horizontal to the east, the positive z axis is downwards vertical. Each observation equipment collects data from three sounding points simultaneously. Whemein, the middle sounding point measures two electric field components and three magnetic field components, while the sounding points at both ends only measure two electric field components. The observation equipment introduced has 12 single channels, including three magnetic channels and nine electrical channels.

In the embodiment of the invention, the performance parameters of the 12 single channels of observation equipment are as follows:

There are 24 frequency sampling points, where the lowest sampling frequency is 24000Hz and the highest is 600 kHz; The bandwidth is DC-i0kHz, the dynamic range is >130dB, the input impedance is >10M 2, and the standard OOMbit twisted-pair cable is used for network connection; The channels support USB 1.1, USB 2.0, wireless, Bluetooth technology, and the synchronization mode is GPS clock, UTC± ns; The internal crystal vibration is <±5x10-9, and the power consumption when 12 channels working simultaneously should be less than 12W.

Embodiment 1

The key contents of the disclosed electromagnetic method for prospecting deeply buried resources in the embodiment are described in detail below.

In the embodiment of the invention, the sky wave signal is generated using the large-scale transmitting moment (transmitting current x antenna length). Noticeably, the electromagnetic waves with SNR greater than 20dB can be observed within 3000km fom the source origin. The theoretical model was eventually verified by the simulation together with the observation position and actual geoelectric structure of the Caosiyao Molybdenum Deposit in the Inner Mongolia Autonomous Region, China. As a result, detailed information of underground geological structum was attained through the observation and dataprocessing oflarge-scale network.

The Caosiyao Molybdenum Deposit, which is the third largest such deposit in China, is located on the northern margin of the North China Craton and associated with the Caosiyao intrusive complex that was emplaced in Archaean metamorphic rocks. Throughout the long geological evolution process, multi- stage fold structure, ductile shear deformation and fault structure wem developed, forming complex structural characteristics for the deposit. One of the large-scale Caosiyao deposits is located 3 km southeast of Chengguan and Xinghe Towns. This deposit is one of the most important metal deposits in the central part of Inner Mongolia. Previous analysis has indicated that the molybdenum resource stored in the Caosiyao Molybdenum Deposit is approximately 2 million tons with total molybdenum output expected to surpass the current outputs in Shapinggou Molybdenum Deposit in Anhui Province and Chalukou Molybdenum Deposit in Heilongjiang Province. Referring to FIG. 5 for the scale, shape and occurrence of the main deposit body proved by drilling. The target orebody is about 2 km long in the east-west direction and 1.6 km long in the north-south direction. The orebody occurs vertically between the elevation of 1267 257 m.

The center of the survey area is located at 40.48 north latitude and 113.55 east longitude. It is about 1200 km away from the large-scale controlled transmitting point. The transmitting power of the emitting point is at 500kW, and the transmitting current is set to be 250A. According to the existing geological data and the shape of the targeted ore body, 7 survey lines together with 651 sounding points wer designed in the area (see parallel lines identified in FIG. 6). The azimuth of the survey line is NW45°, and the length of each survey line is approximately 4.9km with 400m in between lines. The distance in-between sounding points is 50m. As it clearly illustrated in FIG .6, the survey line covers the entire targeted ore body.

On the other hand, 30 observation equipment adopts the positive east-west and North-South mode of pole distribution (shown in FIG. 7). Each observation equipment simultaneously collects data fmm 3 sounding points. The middle sounding point measures 2 electric field components and 3 magnetic field components, while the sounding points set on both sides only measum 2 electric field components. The observation equipment introduced has 12 single channels, including 3 magnetic channels and 9 electrical channels. Referring to Table 1 for the specific performance indicators:

Table 1

12 single channels (3 magnetic channels and

Channel

9 electrical channels)

Bandwidth DC-10kHz

the sample number is 24;

A/D converter the low-fmquency sampling rate at 24000Hz,

the high-frequency sampling rate at 600 kHz

Dynamic range >130 dB

Storage 32G SD card

Input impedance >10M2

standard 100Mbit twisted pair cable, Connection USB 1.1/2.0,wireless, Bluetooth technology

GPS clock, UTC 25ns, Synchronization

the internal crystal vibration is <+5x10-9

Power 12 channels working simultaneously is less than 12W

In the embodiment of the invention, the electromagnetic field distribution of both traditional CSAMT half space model and high-power controlled-source full space model am calculated first using numerical simulation. The results of electric field vector propagation calculated am shown in FIG. 8. In the CSAMT half space model calculation, the length of transmitting source is 2km, the transmitting current is set to be 20A. On the other hand, in the high-power controlled-source full space model calculation, the length of transmitting source is 100km, and the emission curont is set to be 250A. The earth resistivity is 1000ohm.m, the air layer thickness is 100 km, the resistivity is 10 15ohm.m, and the ionosphere resistivity is 10 5ohm.m. Referring now to FIG. 8, an electric field vector prpagation diagram embodying in the posent invention was presented with transmission frequency of 1Hz. In this referred figure, the arrow indicates the direction of the field, and the color ind icates the size of the field value. It can be seen from Fig. 8(a) that the field value calculated by CSAMT method is smaller in amplitude yet faster in attenuation, and the value of frquency domain electric field Ex is only 3.68x10-1 2A/m at the position 1200 km away from the emission source. In contrast, the amplitude of the field value calculated using the high-power controlled-source method has been increased (shown in FIG. 8(b)). The signal generated by the transmitting source can cover the survey area and 100 km beyond. The value of frequency domain electric field Ex at the location 1200 km away from the emission source reaches 1.43x10-8A/m, nearly 3886 times surpassing the result calculated using CSAMT.

FIG. 9 is the spectrum of electric and magnetic field at a site 2,300 m from line 2 embodying in the present invention. The horizontal axis presents the frequency, while the vertical axis presents the electromagnetic field spectrum value, i.e., the amplitude. FIG. 9(a) is a spectrum curve of Ex, FIG. 9(b) is a spectrum curve of Ey, FIG. 9(c) is the spectrum curve of Hx, FIG 9(d) is the spectrum curve of Hy, FIG. 9(e) is the spectrum curve of Hz. Referring now to FIG. 9(a)-(e), it can be clearly seen the relative strength of each frquency signal captured. The high- frquency signal is stronger than the low-frequency signal; frequency that is less than 0.5Hz cannot be identified as it is submerged in the background noise. Furthermore, a specific interference occurs near 200Hz as a small peak can be observed from the curves. This results in a weak signal at frequency 185.81Hz. In each component of electromagnetic field, the signal component of Hz is the weakest, the signal components of others am relatively stinger. The signal component of Ex is stinger than Ey, and Hy is stronger than Hx. This is consistent with the electromagnetic field distribution of the antenna set in North-South direction.

In the embodiment of the invention, the Kania apparent resistivity can be calculated from the observed Ex and Hy. FIG. 10 shows the electric field and the magnetic field curve measured, together with the apparent resistivity and impedance phase curve calculated at 2300m depth of 2 measuring lines. FIG. 10(a) is the electric field amplitude diagram, FIG. 10(b) is the magnetic field amplitude diagram, FIG. (c) is the apparent resistivity curve, FIG. 10(d) is the impedance phase curve. Since the signal at frequency 185.81Hz was interfered, flying points appears on the electric field curve and the magnetic field curve. Yet, smooth curve was obtained on the apparent resistivity curve calculated by electromagnetic and magnetic field, as it has stronger anti- interference ability. The electromagnetic field with a frequency lower than 1Hz has a relatively weak signal, resulting in slightly oscillatory in apparent resistivity curve calculated. Hence, data filtering and processing are needed. In general, the data captured has a relatively good quality, the curve is smooth as expected. Moreover, it can be clearly seen that the apparent resistivity increases as the frequency decreases. This implies that the resistivity of the underground ock is directly proportional to the depth.

FIG. 11 is a pseudo cross-sectional view of the original apparent resistivity and impedance phase of the 3 measuring lines in the xy direction embodying in the present invention. The horizontal axis represents the distance along the survey lines. The vertical axis is the logarithmic coordinate, representing the frquency. FIG. 11(a) is the cross-sectional view of the original apparent resistivity in the xy direction, FIG. 11(b) is the impedance phase pseudo-section in the xy direction. Both FIG. 11(a) and FIG. 11(b) took the logarithm of the frequency and then multiplied by 500 as the coefficient. On the pseudo section of apparent resistivity in xy direction andyx direction of Line03, high resistance characteristics were observed under both 1300 2000m and 3000-4000m sections. Yet, better transverse electrical connectivity was observed on other sounding points. It is not straightforward to determine the geographical and structural information of the targeted ore body based on the apparent resistivity and phase cross-sectional diagrams. Noticeably, the inversion calculation of the data is needed to obtain the underground electrical characteristics.

The full-space nonlinear conjugate gradient (NLCG) method was used to invert the WEM data in three dimensions. The 3-D slice is shown in FIG. 12. Mark the fault F1 and the location of the ore body on the three-dimensional slice. The bedrock on the water-facing side of the lower wall of the Fl fault is fractured, and the fissures have strong water and water richness, rsulting in low resistivity. The black dotted frame in the figure is the position of the om body, and its inversion results are consistent with the ore body: The Caosiyao Molybdenum ore body is formed by the intrusion of underground magma, and the inversion result shows a high-resistance anomaly; the ore body is irregular in shape, and the size of the ore body gradually decreases frm southwest to northeast as indicated in the WEM inversion results. The high-resistance anomaly gradually decreases from line L1 to L5, and line L6 and L7 have no ore body anomalies.

FIG. 13(a) is the inversion rsult using the traditional CSAMT method, FIG. 13(b) is the inversion rsult using the method disclosed. The embodiment of the present invention compares the method disclosed with the traditional CSAMT method, and extracts data that coincides with the location of the geological section from the 3D inversion data volume to obtain large-scale controlled-source resistivity crss sectional diagram (shown in FIG. 13(b)). Refering now to FIG. 13(c), a geological data map is shown. It can be clearly seen that the two methods have a high degree of similarity. The red- yellow high rsistance on the lower part of the figure is the granite body, and the high resistance above the granite body is the targeted ore body. Subsequently, FIG. 13(c) has rvealed that both geological and om bodies am very similar to the inversion profiles of WEM and CSAMT.

The traditional electromagnetic method has the advantages of low power and short dipole moment. According to the prpagation mechanism of formation wave and ground wave, the signal that met the rquirement of signal-to-noise ratio can only propagate approximately 1Okm, which is usually referred as small-scale active-source electromagnetic method. In conducting large-scale observation, the emission devices must be re-arranged multiple times in large-scale observation area since the observation area is restricted (<10km fom the emission source). This ultimately reduces the work efficiency and effectiveness.

The method proposed in the embodiment of the invention is a combination between geophysics and radio physics. It transmits high-power electromagnetic wave by laying a cable with a certain thickness of high resistance layer and a long transmitting electrode distance (usually hundreds of kilometers) and receives the electromagnetic signal to achieve the purpose of deep exploration within thousands of kilometers.

In contrast to the traditional CSAMT method, WEM method uses a transmitting source with fixed current and relatively high power. The distance between the transmitter and oceiver was 1000 km, the transmitting power was 500kW, and the transmitting current was 250 A. In traditional CSAMT method, however, the distance between the transmitter and receiver was only 2km, the transmitting power was 30kW, and the transmitting current was 20A. The modeling result is shown that WEM method has higher transmitting power, higher transmitting current, stronger electromagnetic wave energy and better signal- to-noise ratio of 10dB ~ 20dB in comparison with CSAMT method. The electrical profile of Caosiyao Molybdenum Deposit detected by the "sky wave" is not only consistent with the geological profile, but also showcase better results than the ones detected using the traditional active source method.

In sum, WEM method not only has the characteristics of MT method, such as high depth and low cost, but also has the advantages of active source electromagnetic method (CSAMT, etc.) such as strong anti- interference ability and high detection accuracy. As a newly developed method, WEM method is expected to be used in the exploration of underground and marine sources, earthquake prediction, weather prediction, space physics research, the structure of the earth's sphere and coupling effect and interaction among spheres. Especially in China, WEM is an important technological innovation in underground resource exploration and earthquake prediction. Not only does it have a significant scientific and technological value, but also apracticalvalue and huge potential underthe "Onebelt, One road" imitative.

Claims (6)

Claims

1. A controlled-source audio-frequency magnetotellurics method for prospecting deeply buried resources, comprises:

emitting electromagnetic waves by using transmitter with a fixed current and preset power, and obtaining data on electromagnetic wave through observation equipment profile or matrix;

processing electromagnetic wave data by applying the full-space electromagnetic wave propagation model derived from the attenuation feature of the electromagnetic field in the diffusion medium;

completing a fine detection and attaining detailed geological information of a target ore through data analysis based on the transient electromagnetic field theory.

2. The method according to claim 1, wherein each electric and magnetic field components of the full-space electromagnetic wave propagation model can be expressed as:

Er p 0/ 0 Ids COS n

hD n

,p- o/s-o Ids_ i co0s (P 0 E= JH A(z)Fn H 1 hD 7kOD sin m

EQO y 0 /--o Ids i sin 0 1 hD nkoD I s inO0 nl M

Ids sinIY HS 2m hD n m

Ids isin(o 0 F HO = H n}~ + H m 2m, H hD koD Isin 0 " " M 'A, (z)

Ids i cos p 0 F H =H nFn + - H ''" * hD 7ukoD L n sinm0 , ' A, (Z) where: OP a ] (-cos0), F)aI P(-Cos) whre F"l - 2sin+vn)8 , 2 2sin(v71)8M02

Hn=An(z)S,-2 AnGzs) G,(z), H, =S|A, G, (zs) G, (z) D=aO P is the Legendre polynomial function, A=Z / po /sO,

4 = Z, / pa/ce , H , Hm are the spherical harmonic coefficient;

In the formulas indicated above, A , G , A , S represents the excitation

factors of TM mode and TE mode (with subscript m), high gain function, high normalized sensitivity and propagation factor respectively. Among which, TM mode has subscript n, TE mode has subscript m, the propagation factor is

S= C/V- i5.49a /f. In addition, C is the propagation speed of electromagnetic

wave in vacuum 3.Ox108 m/s, V is the propagation phase speed of electromagnetic wave, f is the electromagnetic wave frequency, , is the attenuation rate of

electromagnetic wave in the earth ionosphere waveguide, po is the permeability, co

is the susceptibility, I is the emission current, ds is the length of the emission dipole,

# is the azimuth of the measuring point calculated from the x-axis, 0 is the angle

between the emission dipole ds and the vector direction of field point.

3. The method according to claim 1, wherein the transmitter with a fixed current and preset power used to emitted electromagnetic waves consists of two transmitting devices:

each transmitting device can emit electromagnetic waves with frequency range of 0.1-300Hz, the length of transmitting antenna is approximately 100 km, power is at 500-1000kW, and current is at 250A.

4. The method according to claim 1, wherein one or more observation equipment profile or matrix to be applied, and each observation equipment profile or matrix to include N number of observation devices:

The N number of observation equipment are set to be fixed with dipoles in the both east-west direction and south-north direction. The positive direction of the x axis is horizontal to the north, the positive y axis is horizontal to the east, the positive z axis is downwards vertical. Each observation equipment collects data from three sounding points simultaneously. Wherein, the middle sounding point measures two electric field components and three magnetic field components, while the sounding points at both ends only measure two electric field components. The observation equipment introduced has 12 single channels, including three magnetic channels and nine electrical channels.

5. The method according to claim 4, wherein the performance parameters of the 12 single channels of observation equipment are as follows: There are 24 frequency sampling points, where the lowest sampling frequency is 24000Hz and the highest is 600 kHz; The bandwidth is DC-10kHz, the dynamic range is >130dB, the input impedance is >1OMQ, and the standard OOMbit twisted-pair cable is used for network connection; The channels support USB 1.1, USB 2.0, wireless, Bluetooth technology, and the synchronization mode is GPS clock, UTC± ns; The internal crystal vibration is <±5x10-, and the power consumption when 12 channels working simultaneously should be less than 12.

Figure. 2 Figure. 1

Emitting electromagnetic waves by using transmitter with a fixed current and 101 preset power, and obtaining data on electromagnetic wave through observation equipment profile or matrix 2020101106

Processing electromagnetic wave data by applying the full-space 102 electromagnetic wave propagation model derived from the attenuation feature of the electromagnetic field in the diffusion medium

Completing a fine detection and attaining detailed geological information of a 103 target ore through data analysis based on the transient electromagnetic field theory

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